Exhaust system

Exhaust manifold (chrome plated) on a car engine

Muffler, and tailpipe on a car

An exhaust system is used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system design, the exhaust gas may flow through one or more of:

• Cylinder head and exhaust manifold
• A turbocharger to increase engine power.
• A catalytic converter to reduce air pollution.
• A muffler (North America) / silencer (UK/India), to reduce noise.

Terminology

Manifold or header

Aftermarket exhaust manifold

In most production engines, the manifold is an assembly designed to collect the exhaust gas from two or more cylinders into one pipe. Manifolds are often made of cast iron in stock production cars and may have material-saving design features such as using the least metal, to occupy the least space necessary, or have the lowest production cost. These design restrictions often result in a design that is cost-effective but that does not do the most efficient job of venting the gases from the engine. Inefficiencies generally occur due to the nature of the combustion engine and its cylinders. Since cylinders fire at different times, exhaust leaves them at different times, and pressure waves from gas emerging from one cylinder might not be completely vacated through the exhaust system when another comes. This creates back pressure and restriction in the engine's exhaust system that can restrict the engine's true performance possibilities.

Regardless of the negative attributes of steel tube exhaust outlet configurations, engineers who design engine components choose conventional cast iron exhaust manifolds list positive attributes, such as an array of heat management properties and superior longevity to any other type of exhaust outlet design.

A header is a manifold specifically designed for performance. During design, engineers create a manifold without regard to weight or cost but instead for optimal flow of the exhaust gases. This design results in a header that is more efficient at scavenging the exhaust from the cylinders. Headers are generally circular steel tubing with bends and folds calculated to make the paths from each cylinder's exhaust port to the common outlet all equal length and joined at narrow angles to encourage pressure waves to flow through the outlet, not back towards other cylinders. In a set of tuned headers the pipe lengths are carefully calculated to enhance exhaust flow in a particular engine revolutions per minute range.

A common method of increasing the power output of an engine is the use of upgraded headers. The increased power output is often due to a result of a larger cross-section area of the pipes (reducing the resistance on the exhaust gasses) and/or designing the pipe lengths so that the pressure wave assists in exhaust scavenging. For inline-four engines and V8 engines, exhaust manifolds are usually either a 4-2-1 design (where the four pipes merge into two, followed by a separate merge of these two pipes into one) or a 4-1 design (where the four pipes directly merge into one).

Headers are generally made by aftermarket automotive companies, but sometimes can be bought from the high-performance parts department at car dealerships. Generally, most car performance enthusiasts buy aftermarket headers made by companies solely focused on producing reliable, cost-effective well-designed headers specifically for their cars. Headers can also be custom designed by a custom shop. Due to the advanced materials that some aftermarket headers are made of, this can be expensive. Luckily, an exhaust system can be custom-built for any car, and generally is not specific to the car's motor or design except for needing to properly connect solidly to the engine. This is usually accomplished by correct sizing in the design stage, and selecting a proper gasket type and size for the engine.

Catalytic converter

Some systems (referred to as catless or de-cat systems) eliminate the catalytic converter. It is a U.S. legal requirement to have a catalytic converter. Converters may not be removed from a vehicle that is used only for "off-road" driving in the United States. The main purpose of a catalytic converter on an automobile is to reduce harmful emissions of hydrocarbons, carbon monoxide, and nitrogen oxides into the atmosphere. They work by transforming the polluted exhaust components into water and carbon dioxide. There is a light-off temperature from which catalytic converters start to be efficient and work properly.

Catalytic converters can cause back pressure if they are not designed for the required flow rate or if they are clogged. In these situations, upgrading or removal of the catalytic converter can increase power at high revs. However, the catalytic converter is a key component of the vehicle's emission control systems, therefore a non-standard product can cause a vehicle to be unroadworthy.

Piping

The piping that connects all of the individual components of the exhaust system is called the exhaust pipe. If the diameter is too small, power at high RPM will be reduced. Too large a diameter can reduce torque at low RPM and can cause the exhaust to sit lower to the ground, increasing the risk of it being hit and damaged while the car is moving.

On cars with two sets of exhaust pipes, a crossover pipe is often used to connect the two pipes. Common designs of crossover pipes are a perpendicular pipe ('H-pipe', due to their shape) or angled pipes that slowly merge and separate ('X-pipe').

Muffler

Muffler installed in a car (view from rear of muffler)

Original equipment mufflers typically reduces the noise level from the tailpipe by bouncing sound waves off of the back, front, and sides of the muffler. They are designed to meet the maximum allowable noise level required by government regulations, however, some original equipment mufflers are a significant source of backpressure.

Glasspack mufflers (also called 'cannons' or 'hotdogs') are straight-through design mufflers that consist of an inner perforated tube, an outer solid tube, and fibreglass sound insulation between the two tubes. They often have less back pressure than original equipment mufflers, but are relatively ineffective at reducing sound levels. Another common type of muffler is the chambered muffler, which consists of a series of concentric or eccentric pipes inside the expansion chamber cavity. These pipes allow sound to travel into them and cause the sound waves to bounce off the closed, flat, ends of the pipe. These reflections partially cancel each other out, reducing the sound level.

Resonators are sections of pipe that expand to a larger diameter and allow the sound waves to reflect off the walls and cancel out, therefore reducing the noise level. Resonators can be used inside mufflers, or also as separate components in an exhaust system.

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Header-back

The Header-back (or header back) is part of the exhaust system from the outlet of the header to the final vent to open air — everything from the header back. Header-back systems are generally produced as aftermarket performance systems for cars without turbochargers.

Turbo-back

The Turbo-back (or turbo back) is the part of the exhaust system from the outlet of a turbocharger to the final vent to open air. Turbo-back systems are generally produced as aftermarket performance systems for cars with turbochargers. Some turbo-back (and header-back) systems replace stock catalytic converters with others having less flow restriction.

Cat-back

Cat-back (also cat back and catback) refers to the portion of the exhaust system from the outlet of the catalytic converter to the final vent to open air. This generally includes the pipe from the converter to the muffler, the muffler, and the final length of pipe to open air.

Cat-back exhaust systems generally use larger diameter pipes than the stock system. The mufflers included in these kits are often glasspacks, to reduce backpressure. If the system is engineered more for show than functionality, it may be tuned to enhance the lower sounds that are lacking from high-RPM low-displacement engines.

Exhaust aftertreatment

Exhaust aftertreatments are devices or methods used to meet emission regulations.

• Catalytic converter
• Exhaust gas recirculation (EGR)
• Diesel particulate filter (DPF)
• Diesel exhaust fluid (DEF or AdBlue)
• Carbon capture and storage
• Selective non-catalytic reduction (SNCR)
• Scrubber

Exhaust system tuning

Aftermarket exhaust system including headers and a white plasma-sprayed ceramic coating

Aftermarket exhaust parts can increase peak power by reducing the back pressure of the exhaust system. These parts sometimes can void factory warranties, however the European Union Block Exemption Regulations 1400/2002 prevents manufacturers from rejecting warranty claims if the aftermarket parts are of matching quality and specifications to the original parts.

Many automotive companies offer aftermarket exhaust system upgrades as a subcategory of engine tuning. This is often fairly expensive as it usually includes replacing the entire exhaust manifold or other large components. These upgrades however can significantly improve engine performance by reducing the exhaust back pressure and by reducing the amount of heat from the exhaust being lost into the underbonnet area. This reduces the underbonnet temperature and consequently lowers the intake manifold temperature, increasing power. This also has a positive side effect of preventing heat-sensitive components from being damaged.

Backpressure is most commonly reduced by replacing exhaust manifolds with headers, which have smoother bends and normally wider pipe diameters.

Exhaust heat management helps in reducing the amount of exhaust heat radiated out from the exhaust pipe and components. One dominant solution to aftermarket upgrades is the use of a ceramic coating applied via thermal spraying as a heat shield. This not only reduces heat loss and lessens back pressure, but also provides an effective way to protect the exhaust system from wear and tear, thermal degradation, and corrosion.

Tuning can change the noise of the exhaust system, known as exhaust notes.

Images

• 
  Large truck's diesel exhaust pipe
• 
  Waste collection vehicle's diesel exhaust pipe
• 
  Dual exhaust pipes attached to a car's muffler
• 
  Exhaust system of diesel telescopic-arm vehicle
• 
  Underbody of a car showing the exhaust system

See also

• Vehicle emissions control
• Expansion chamber
• Motor vehicle emissions
• Nitrogen oxide sensor
• British Leyland Motor Corp v Armstrong Patents Co - litigation involving right to supply aftermarket exhaust systems
• Exhaust Heat Management
• Zircotec

External links

• Media related to Exhaust systems at Wikimedia Commons